1. Field of the Invention
The present invention relates to a lateral insulated gate bipolar transistor (lateral IGBT).
2. Description of the Related Art
Japanese Patent No. 3,522,983 (corresponding to U.S. Pat. No. 5,731,603 and referred to as a patent document 1 hereafter) discloses a lateral IGBT in which the number of channels is increased to reduce an on-voltage. The patent document 1 also discloses that preventing operation of a parasitic thyristor is important to improve a latch-up immunity. This is because a current density in silicon in a lateral IGBT is much higher than a current density in silicon in a vertical IGBT. In a vertical IGBT, because electric current flows in a normal direction of a substrate, a cross-sectional area in which electric current flows is large, and a current density is not high. In a lateral IGBT, because electric current flows in a horizontal direction of a substrate, a cross-sectional area in which electric current flows depends on a thickness of the substrate, and a current density is high.
In a vertical IGBT, as disclosed in Japanese Patent No. 3,395,520 (referred to as a patent document 2 hereafter), a resistance of a silicon layer is reduced by forming an high impurity concentration region around an emitter region. A similar technique is disclosed in Japanese Unexamined Patent Application Publication No. 10-178174 (corresponding to Europe Unexamined Patent Application Publication No. 0387508A2 and referred to as a patent document 3 hereafter), and the above-described technique is applied to a lateral IGBT. In Japanese Unexamined Patent Application Publication No. 2008-270377 (corresponding to US Unexamined Patent Application Publication No. 2008/0265278 A1 and referred to as a patent document 4 hereafter), an example in which the above-described technique is applied to a lateral IGBT having a different structure. The patent document 4 discloses a technique that focuses on a specialty of a lateral IGBT and a short circuit capacity as one of breakdown endurance can be improved by controlling a gate electrode.
Because a current density in a lateral IGBT at operation is much higher than a current density in a vertical IGBT at operation, the lateral IGBT has difficulty in securing a breakdown endurance. Furthermore, due to a high current density, an on-state breakdown voltage and a transitive-state breakdown voltage are reduced, and a switching speed is reduced. Thus, a local dynamic avalanche easily occurs in a lateral IGBT at switching compared with a vertical IGBT. Therefore, there is a trade-off relationship between reduction of an on-voltage and extension of a turning-off time, an on-state breakdown voltage and a transitive-state breakdown voltage. It is important to achieve high trade-off balance.
In order to reduce an on-voltage of a lateral IGBT, a carrier concentration in an n− type drift layer in the vicinity of an emitter is increased. Thus, it is required to restrict holes from flowing to a channel p well layer and to facilitate accumulation of carriers. In other words, because the on-voltage increases when the carrier concentration in the n− type drift layer in the vicinity of the emitter decreases, it is important to increase the carrier concentration in the n-type drift layer in the vicinity of the emitter. A method of increasing the carrier concentration in the n− type drift layer in the vicinity of the emitter includes (i) narrowing the channel p well layer so as to narrow a region from which holes are extracted or (ii) arranging an n type hole barrier layer between the channel p well layer and the n− type drift layer so that extraction of holes is restricted.
Although the patent document 2 discloses that forming a high impurity concentration region around an emitter region can be applied to a lateral IGBT also, a specific application structure of the lateral IGBT is not suggested. This method corresponds to the above-described method (ii), the patent document 2 discloses only a DC characteristic, and it is not clear how to apply the above-described method (ii) to the lateral IGBT. The patent documents 2 and 3 are based on a vertical IGBT and do not take measures against that the current density in the lateral IGBT is much higher than the current density in the vertical IGBT. The patent document 4 only contrives a control method of a gate electrode and cannot cancel the tradeoff relationship with a demerit of increasing an on-voltage.